GAS FOIL BEARINGS FOR OILFREE ROTATING MACHINERY

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28th Turbomachinery Research Consortium Meeting
Development of a Test Rig for Metal Mesh Foil Gas
Bearing and Measurements of Structural Stiffness
and Damping in the Metal Mesh Foil Bearing
Luis San Andrés Tae-Ho Kim Thomas Abraham
Chirathadam Alex Martinez
Project title Metal Mesh-Top Foil Gas Bearings
for Oil-Free Turbomachinery Test Rig for
Prototype Demonstration
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TAMU past work on Metal Mesh Dampers
METAL MESH DAMPERS proven to provide large
amounts of damping. Inexpensive. Oil-free
Zarzour and Vance (2000) J. Eng. Gas Turb.
Power, Vol. 122 Advantages of Metal Mesh Dampers
over SFDs Capable of operating at low and high
temperatures No changes in performance if soaked
in oil
Al-Khateeb and Vance (2001) GT-2001-0247 Test
metal mesh donut and squirrel cage( in
parallel) MM damping not affected by modifying
squirrel cage stiffness
Choudhry and Vance (2005) Proc. GT2005 Develop
design equations, empirically based, to predict
structural stiffness and viscous damping
coefficient
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Recent Patents gas bearings systems
Air foil bearing having a porous foil Ref.
Patent No. WO 2006/043736 A1
A metal mesh donut is a cheap replacement to
porous foil
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TRC Project Tasks 07/08

• Construction of Metal Mesh Foil Bearings
• -Assembly of top foil and metal mesh donut inside
  a cartridge
• Identification of structural force coefficients
• -Static load-deflection tests for structural
  stiffness
• -Dynamic load tests for stiffness and structural
  loss factor
• -Effects of frequency
• Construction of test rig for demonstration of
  MMFB Performance
• -Turbocharger (TC) driven system

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Metal Mesh Foil Bearing (MMFB)
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Metal Mesh Foil Bearings

• Metal mesh donut and top foil assembled inside a
  bearing cartridge.
• Hydrodynamic air film will develop between
  rotating shaft and top foil.

• Metal mesh resilient to temperature variations
• Damping from material hysteresis

• Stiffness and viscous damping coefficients
  controlled by metal mesh material, size
  (thickness, L, D), and material compactness
  (density) ratio.

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Metal Mesh Foil Bearings (/-)

• No lubrication (oil-free). NO High or Low
  temperature limits.
• Resilient structure with lots of material
  damping.
• Simple construction ( in comparison with other
  foil bearings)
• Cost effective

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MMFB dimensions and specifications
PICTURE
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Static load test setup
Lathe chuck holds shaft bearing during
loading/unloading cycles.




Eddy Current sensor


Stationary shaft
Lathe tool holder
Test MMFB
Lathe tool holder moves forward and backward
push and pull forces on MMFB
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Static Load vs bearing deflection results
MMFB wire density 20
3 Cycles loading unloading

Nonlinear F(X) Large hysteresis loop Mechanical
energy dissipation
Displacement -0.06,0.06 mm Load -130, 90 N
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Derived MMFB structural stiffness
MMFB wire density 20

During Load reversal jump in structural
stiffness
Max. Stiffness 4 MN/m
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Dynamic load tests
Motion amplitude controlled mode
12.7, 25.4 38.1 µm
Frequency of excitation 25 400 Hz (25 Hz
interval)
Waterfall of displacement
MMFB motion amplitude (1X) is dominant.
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Amplitude of Dynamic Load vs Excitation Frequency
Dynamic load decreases with increasing
frequency and decreasing motion amplitudes
Motion amplitude decreases
At higher frequencies, less force needed to
maintain same motion amplitudes
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Identification Model
1-DOF mechanical system
Equivalent Test System
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Parameter Identification (no shaft rotation)
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Real part of (F/X) vs excitation frequency
Frequency of excitation 25 400 Hz ( 25 Hz
step)
Motion amplitude increases
Real part of (F/X) decreases with increasing
motion amplitude
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MMB structural stiffness vs excitation frequency
Frequency of excitation 25 400 Hz (25 Hz
step)
K
At low frequencies (25-100 Hz), Stiffness
decreases fast. At higher frequencies,
Stiffness levels off
Motion amplitude increases
MMFB stiffness is frequency and motion
amplitude dependent
Al-Khateeb Vance model reduction of stiffness
with force magnitude (amplitude dependent)
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Imaginary part of impedance (F/X) vs frequency
Frequency of excitation 25 400 Hz ( at 25 Hz
interval)
Motion amplitude increases
Im (F/X) decreases with motion amplitude, little
frequency dependency
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Loss factor vs excitation frequency
Frequency of excitation 25 400 Hz ( at 25 Hz
step)
Structural damping or loss factor increases
with frequency ( 25-150 Hz) But, remains
nearly constant for higher frequencies ( 175-400
Hz)
Loss factor frequency independent at high freqs.
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Model of Metal Mesh damping material
Stick-slip model (Al-Khateeb Vance, 2002)
Stick-slip model arranges wires in series
connected by dampers and springs.
As force increases, more stick-slip joints
between wires are freed, thus resulting in a
greater number of spring-damper systems in
series.
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Design equation Metal mesh stiffness/damping
Empirical design equation for stiffness and
equivalent viscous damping coefficients
(Al-Khateeb Vance, 2002)
Functions of equivalent modulus of elasticity
(Eequiv), hysteresis coeff. (Hequiv), axial
length (L), inner radius (Ri), outer radius (Ro),
axial compression ratio (CA), radial interference
(Rp), motion amplitude (A), and excitation
frequency (?)
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Stiffness prediction test data
MMFB structural stiffness decreases as frequency
increases and as motion amplitude increases
12.7 µm
25.4 µm
38.1 µm
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Predictions compared to test data Damping
MMFB equiv. viscous damping decreases as the
excitation frequency increases and as motion
amplitude increases
12.7 µm
25.4 µm
38.1 µm
Predicted equivalent viscous damping coefficients
in good agreement with measurements
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Metal Mesh Foil Bearing Rotordynamic Test Rig
(a) Static shaft
TC cross-sectional view Ref. Honeywell drawing
448655
Max. operating speed 120 krpm Turbocharger
driven rotor Regulated air supply9.30bar (120
psig)
Twin ball bearing turbocharger, Model T25,
donated by Honeywell Turbo Technologies
Test Journal length 55 mm, 28 mm diameter ,
Weight0.22 kg
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Metal Mesh Foil Bearing Rotordynamic Test Rig
Static load

Load cell
Eddy current sensor


Torque arm
TC driving system
Weight
Rotating journal
Squirrel cage

Spring

Positioning table

(a) Right side view
(b) Front view
Static load applies upwards using weights
pulleys Arm and load cell to measure bearing
torque measurement
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Metal Mesh Foil Bearing Rotordynamic Test Rig

• Squirrel Cage
• Provides soft support to MMFB
• Maintains concentricity (prevents tilting) of
  MMFB with test journal

• Positioning table
• Max load 110N
• Max 3X 3 travel in two directions
• Resolution of 1µm
• Supports squirrel cage
• Provides motion in two horizontal directions

COST of positioning TABLE 3631
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Conclusions

• TC driven MMFB rotordynamic test rig under
  construction
• Static and dynamic load tests on metal mesh
  bearings show large energy dissipation and
  (predictable) structural stiffness
• MMFB stiffness decreases with amplitude of
  dynamic motion
• Large MMFB structural loss factor ( g 0.50 ) at
  high frequencies

Predicted stiffness and equivalent viscous
damping coefficients are in agreement with test
coefficients Test data validates design equations
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TRC Proposal Metal Mesh Foil Bearings for
Oil-Free Turbo-machinery Rotordynamic
performance
Complete construction of turbocharger driven
MMFB test rig squirrel cage, static loading
device and torque measurement device Conduct
experiments on test rig Rotor lift off and
touch down speeds, measurements of torque load
capacity, vibration and stability (if any)
Identification of dynamic force response
Impact loads on test bearing more measurements
of structural stiffness and loss factor
TASKS